Optical communication systems operate by transmitting a modulated laser signal over a span of optical fibre before detection of the signal at a receiver. If the optical signal is too weak to be detected at the receiver, optical amplification can be used to boost the power of the signal and hence compensate for the loss of the fibre span. However, this amplification adds noise onto the optical signal and thus degrades the optical signal to noise ratio (OSNR). If the number of spans and amplification stages is large before receiving then the OSNR can be degraded to such an extent that the signal is difficult to recover. This is exacerbated at high data rates. One way of overcoming this degradation is to simply boost the signal power at the transmitter and at the start of every subsequent fibre span. At low signal powers in what is known as the ‘linear regime’ of the fibre this is possible and accordingly boosts the OSNR and hence improves the received signal quality. However, there exists a signal power threshold at which increased optical power can actually cause degradation in the signal quality even though the OSNR is still improving. This is known as the nonlinear threshold and the regime above this power known as the ‘nonlinear regime’ of the fibre.
Techniques exist for mitigating the nonlinear degradation of an optical signal in often complex and expensive ways. For example US 2004/0197103A1 discloses a technique for pre-distorting a signal before transmission in order to take into account the effect of the nonlinearities and dispersion of the fibre, so that after these effects have occurred on the signal during its passage through the fibre, the original signal has been recovered. This technique requires high speed electronics in order to operate at the high signal data rates involved, and is hence expensive. It also requires details of the configuration of the optical path across which the optical signal is to be transmitted.